Real Time Control of the Active MHD Diagnostic on Alcator C Mod

An active MHD diagnostic has been installed and operated on Alcator C-Mod to study Toroidal Alfven Eigenmodes (TAEs). The diagnostic consists of two broadband amplifiers and two antennas which can excite stable modes in the TAE frequency range. The plasma response to this excitation is measured by existing magnetic fluctuation diagnostics. Analysis of the fluctuation data provides information about the TAE frequencies and damping rates as well as mode structure. The active MHD diagnostic has evolved since its first operation in 2002, when it could only operate over a limited, preprogrammed frequency band. Recent improvements include automatic tuning to cover frequencies from 100 to 750 kHz, and an interface to the Alcator C-Mod digital plasma control system (DPCS). The DPCS monitors plasma parameters, calculates the nominal TAE frequency, and adjusts the frequency input to the MHD amplifiers, all in real time. Now the active MHD diagnostic can measure TAEs over an entire plasma shot. This paper is focused on the requirements and design of the MHD amplifiers, particularly the tuning bandwidth. It will also present some preliminary results from the recent run campaign